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[Tim Perry]

Go back and re-read what Hal wrote.  You apparently don't understand 70V systems either.

The power rating of the 70V amplifier is irrelevant, as long as it's higher than the total load of the speakers.  Your conclusion is correct, but the path that lead you there appears to be rather meandering. 

GTD

Quite simply Hals analogy is dead wrong.  A generator provides a constant voltage. An audio amplifier provides a varying voltage. The larger the amp the larger the Vmax.  At some point you reach the ability to fry the transformers and/or voice coils.

Take the same 'generator' and use the 220v outlet instead of 110 and see how long the lightbulbs last. 

[Geoff Doane]

Great!  Now I've lost it.  I'm arguing on the internet. 

Quite simply Hals analogy is dead wrong.  A generator provides a constant voltage. An audio amplifier provides a varying voltage. The larger the amp the larger the Vmax.  At some point you reach the ability to fry the transformers and/or voice coils.

Take the same 'generator' and use the 220v outlet instead of 110 and see how long the lightbulbs last.

Hal's analogy included a dimmer.  He didn't say anything about a 220V outlet.  He merely said the generator was capable of 600W and the light bulbs would only draw 300W.  Since he didn't mention anything about voltage, I think it's safe to assume that the light bulbs and the generator both operate at the same voltage (leaving the dimmer out of it for a minute).

And that's exactly how the amplifier works.  The maximum undistorted voltage out of a 25W  or 600W 70V amplifier is 70V.  The practical difference is how many 25W-tapped speakers you can connect to each one before the amp goes into current limiting. 

I've long been suspicious that 70V systems were invented because they are usually installed by electricians, and electricians understand how to hook up strings of light bulbs. Just substitute speakers with transformers for light bulbs, and install an amplifier of adequate power, and you're in business.

GTD

[Brad Weber]

I think one possible point of confusion is whether we are looking at an amp with a dedicated 70V output or a low impedance amp being used to try to direct drive a 70V line.  Simple Ohm's Law that basically any amp rated 625W or higher into 8 Ohms (1227W into 4 Ohms) can be used to direct drive a 70V system but below that the amp needs an output transformer to step up the voltage.

I bring this up as the choice of amplifier may have more to do with wanting enough output for the connected speakers and transformers, plus some for line losses, while also being able to provide a 70V (actually 70.7V) output.  Would a 350W or 400W amp be sufficient for 300W of load, probably, but you may not find that many cost effective 350W or 400W rated amps with 70V outputs.  But a 625W amp could provide plenty of power for the load and direct drive it as a 70V system.

70V (and 100V, 140V, etc.) systems were developed greatly to reduce cable losses where you had very long runs, much like why power lines are stepped up to high voltage and then stepped back down at the endpoint.  This could definitely be a factor a venues like a stadium, airport or raceway.

[Hal Bissinger/COMSYSTEC]

I've long been suspicious that 70V systems were invented because they are usually installed by electricians, and electricians understand how to hook up strings of light bulbs. Just substitute speakers with transformers for light bulbs, and install an amplifier of adequate power, and you're in business.

Actually it was invented to eliminate impedance calculations. Just add up the taps and use an amplifier with more power.
 
-Hal

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